It is difficult to maintain a single spatial mode of operation and a coherent single far-field diffraction-limited lobe in semiconductor lasers, amplifiers and MOPA devices to high output powers. In semiconductor lasers it has been found necessary to keep the waveguide width to less than 5 .mu.m. Otherwise, multimode oscillation can occur at high pumping levels. Single stripe lasers are limited to single mode outputs of approximately 200 mW cw. Significantly higher powers are available from broad area lasers. For example, reliable operation to 1.0-1.5 W cw is available in broad area lasers with 100 .mu.m emitting apertures. Unfortunately, such broad area lasers typically operate in multiple spatial modes, so the resulting beam is not diffraction limited.
High diffraction-limited powers have been obtained by injecting discrete broad area traveling wave amplifiers with a high power coherent beam from an external Ti:sapphire laser or dye laser. Using a Ti:sapphire laser as the master oscillator to inject a 0.5 W pulsed beam into a discrete broad area amplifier, an amplified output of greater than 21 W pulsed with a single diffraction-limited lobe was achieved. Fully monolithic structures replacing the external laser source with one that is integrated on the same substrate as the optical power amplifier have been demonstrated. The most successful monolithic MOPA structures to date have been those with a flared amplifier whose width increases from a narrow entrance aperture at the emitting end of the single mode laser oscillator to a significantly wider exit aperture. To preserve the single spatial mode received from the laser oscillator, the light is allowed to freely diffract within the flared amplifier, with the rate of increase of amplifier width coinciding with the divergence of the beam. Further, the intensity of the light received from the laser should be near the saturation intensity of the amplifier, with the power increasing along the length of the amplifier in a manner that maintains the intensity at or above the saturation level.
In U.S. Pat. No. 4,063,189, Scifres et al. describe a heterojunction diode laser that uses leaky wave coupling of light out of the laser active region through a thin confining layer to produce a high power, single transverse mode, large area, low divergence output beam. Several embodiments place the thin confining layer parallel to the active region layer, so that light is leaked transversely into the substrate at an angle to the rectifying junction of the laser. The substrate has a composition or heavy dopant concentration selected to minimize absorption at the lasing wavelength. In another embodiment, the thin optical confinement layer is located on the side of the active waveguide core and oriented perpendicular to the plane of the active region. In this latter embodiment, the light leaks laterally out of the waveguide. Because the optical power is extracted from an extended length of the active region, the output beam is spread out over a large area of the emitting surface of the laser diode.
In U.S. Pat. No. 3,753,157, Ash et al. describe a leaky wave coupler for transferring optical waves from one waveguide to another. The waveguides are two parallel longitudinal regions spaced apart from one another on the surface of a bulk material. The bulk material may be a slab of fused quartz. The waveguides may be lossless dielectric material, such as borosilicate glass, having a refractive index greater than that of the bulk material. A coupling region is positioned on the surface of the bulk material between the two waveguides in an area where coupling is desired. The coupling region may be a thicker layer of borosilicate glass than either waveguide, and thus has an even greater effective index of refraction than the two waveguides. Optical waves propagating in one waveguide approach that section of the waveguide which is adjacent to the coupling region. The lower phase velocity characteristic of the coupling region causes the waves to gradually leak from the waveguide into the coupling region. The leaky waves then traverse the coupling region and are coupled into the second waveguide in a reverse, but similar, manner. The result is strong coupling between the two waveguides even when the waveguides are not closely spaced. In another embodiment, the interguide region in the area where coupling is desired is a film layer that has a refractive index greater than that of the bulk material, but less than that of both waveguides. Sections of the waveguides are made with reduced refractive index, such as thin layers of borosilicate glass compared to the thicker layers in the remainder of the waveguides, so that light waves leak from one of these sections into the interguide film region and then into the corresponding section of the other waveguide. In yet another embodiment, both the waveguides and the coupling region between the waveguides could be made of an electro-optic material, such as LiNbO.sub.3. Electrodes connected to the outside bulk material above and below the coupling region apply a controlled bias voltage in the coupling region. An electric field of one polarity in the electro-optic material increases the refractive index of the coupling region so that optical waves leak out of one waveguide and are conducted across the region to the other waveguide, while an electric field of the opposite polarity decreases the refractive index so as to prevent leaky wave coupling of the waveguides. Ash et al. also describe various leaky wave surface-to-bulk couplers using similar principals.
Ackley et al., in U.S. Pat. No. 4,348,763, and Botez et al., in U.S. Pat. No. 4,985,897, describe multistripe leaky mode or antiguided semiconductor laser arrays. In the lasers described by Ackley et al., multiple active stripe regions are separated by passive coupling regions. The passive coupling regions have a refractive index that is higher than the effective refractive index of the active stripe regions, so radiation leaks out of the stripe regions into the passive regions at a shallow angle, thereby providing optical coupling between neighboring stripe regions. In the lasers described by Botez et al., an array of negative-index waveguides or "antiguides" form the active lasing elements which are separated by interelement regions of higher refractive index. Some of the light propagating in the lower index antiguide elements will leak out into the interelement regions. The lateral spacing between active antiguide elements is selected to approximately equal an odd number of half-wavelengths in order to produce an array mode resonance condition in which there is strong coupling between all elements of the array, thereby creating a high degree of device coherence and a practically uniform near-field intensity distribution across the array. The lasers described by Botez et al. feature a monolithic structure of two such arrays longitudinally spaced apart on opposite ends of the structure and separated by a Talbot length, laterally unguided diffraction region. The two arrays are arranged relative to one another so as to operate at high power in a selected array mode (either in-phase or out-of-phase).
Scifres et al., in U.S. Pat. No. 4,815,084 describe a semiconductor laser with one or more optical elements integrated within the structure by means of boundaries of refractive index changes that are shaped and oriented to produce changes in shape of phase fronts of lightwaves propagating across the boundaries within the laser. In one embodiment, a boundary between the laser active region and a transparent window region is planar, but oriented askew, i.e. not parallel, relative to the cleaved end facet. Thus, the active region defines a prism, which can be used to deflect the laser beam, causing beam expansion.
An object of the present invention is to provide a monolithic broad area MOPA device capable of generating diffraction-limited outputs greater than 1.0 W cw.
Another object is to provide semiconductor laser oscillators capable of generating a broad area single spatial mode output at high power levels, as well as providing broad area traveling-wave optical power amplifiers capable of using their full width over their entire length for amplifying coherent light received from a laser source.